1. Field of the Invention
This invention relates to protective coating compositions. More particularly, this invention relates to aqueous oligomeric silicon coating compositions which when applied to various substrates provide a hard corrosion resistant coating. The compositions which do not include pigment are substantially transparent. The invention also relates to the coated articles and to the methods for coating articles using the aqueous oligomeric silicon coating composition as well as to the method for forming the coating compositions.
2. Discussion of Prior Art
The closest prior art known to Applicant includes four patents issued to Dow Corning in the name of H. A. Clark. These patents include U.S. Pat. Nos. 3,986,997; 4,027,073; 3,976,497; and 3,944,702. The disclosures of these patents are incorporated herein in their entireties by reference thereto. The '997 and '073 patents describe coating compositions primarily intended to provide a hard protective surface to soft plastic sheet material, especially eyeglass lenses. Coating of metal, including aluminum, is also described. These coating compositions are acidic dispersions of colloidal silica and hydroxylated silsesquioxane in an alcohol-water medium. The coating compositions of '497 and '702 also include colloidal silica.
The present invention is based, in part, on the discovery that colloidal silica, especially when used in or near the amounts contemplated by the above Dow Corning (Clark) patents, renders the coatings porous or microporous and drastically reduces the corrosion resistance of the coatings.
The present invention overcomes these and other drawbacks of known corrosion and abrasion resistant silicon-based coating compositions.